Nitric acid treatment of steel



NITRIC ACID TREATMENT OF STEEL Paul W. Hardy, Barrington, Ill., assignor to American Can Company, New York, N.Y., a corporation of New Jersey No Drawing. Application June 27, 1956 Serial No. 594,112

Claims. (Cl. 134-3) The present invention pertains to the chemical treatrneptgf the surface of a ferrous metal and'i'n partifil'la'r to the treatment of ferrous metal surfaces with dilute, a ueous nitric acid.

An object of the present invention is to provide a method of treating a corrodible ferrous metal to form on the surface thereof a characteristic pattern indicating the areas of subsequent atmospheric corrosion or rustmg.

Another object is to provide a method of treating a corrodible ferrous metal to permit, by means of visual inspection, a qualitative and roughly quantitative determination of the corrodibility or rust-potential thereof at the time of its fabrication and long before the rusting normally occurs.

A further object is to provide a method of treating a corrodible ferrous metal which treatment will inhibit subsequent atmospheric corrosion or rusting thereof.

Numerous other objects and advantages of the invention will be apparent as it is better understood from the following description which is of a preferred embodiment thereof.

The present invention is described hereinafter in relation to low carbon steel, such as that used in the fabrication of sheet metal cans. This type of steel is known in the art as black iron or CMQ (can makers quality) plate. However, it is to be understood that present invention is operable on iron surfaces and other types of corrodible steel similar to black plate.

The Qp centrati0n of the nitric acid so d hereinafter is expressed as percent by volume. For example, a 1% nitric acid solution would contain 1 ml. of concentrated nitric acid per 100 ml. of solution. In making these solutions, commercial concentrated nitric acid was used containing 69-71% by weight HNO and having a specific gravity of l.4l6-l.426 at 60 F.

I have discovered that by contacting a black iron surface with a dilute, aqueous solution of nitric acid for a brief period of time, thereafter rinsing the surface to remove any solution remaining thereon and then drying the surface, the phenomenon showed that the stain indicated the areas of the plate surface in which subsequent rusting would occur if the plate were not treated to produce the stain. It was also observed that the areas developing the darker stain indicated the areas in which greater rusting would occur on untreated plate. In other words, the stain produced on a black plate surface by the method of the present invention affords a means of predicting where and, to some extent, to what degree a black plate surface is liable to rust. An additional result achieved by the instant method is the passivation of the black plate surface whereby subsequent rusting of the surface is inhibited.

Both the time of treatment and the concentration of nitric acid in the treafig solution are critical. Expefi mellmtliaffhmain formed on the sheet is readily removed if the nitric acid concentration is too high or if the sheet remains in contact lH-l Lin-trulwith the treating solution too long, whereby etching of the plate occurs. Generally speaking, the nitric acid concentration must be sufliciently low and the treating time sufliciently short to prevent etching of the plate surface.

Since the rate of the reaction, taking place during the aqueous nitric acid treatment of the black iron, varies directly with the temperature of the treating solution in contact with the black iron, the time of treatment can be varied by varying the treating solution temperature. However, since it permits practicable nitric acid concentrations and treating times and also obviates the need of costly and more complicated apparatus, i.e. heating or cooling means, a treating solution at or about room temperature, L960? F. to F. is preferred.

The time of treatment varies inversely with the nitric acid content of the solution, i.e. the higher the nitric acid concentration, the less the time of contact between the black iron sheet and solution to form but not remove the required stain and vice versa. For this reason, the upper limit of the concentration is governed by the time element. In other words, the nitric acid concentration of the treating solution must not be so high as to make the time limit so short that it is substantially physically impossible to make and discontinue contact between the sheet and the solution without removing the stain formed. The controlling factor on the lower limit of the nitric acid concentration is the ability to maintain consistently a very low nitric acid concentration. Since a change in concentration necessitates a change in contact time to form and preserve the stain for visual observation, this low concentration must be maintainable with a reasonable degree of accuracy.

I have found that a nitric acid concentration of about 11.95% of the aqueous solution, requiring a treating time of about 60 seconds, to be an operable minimum. It is recognized that solutions containing lower concentrations of nitric acid, allowing longer treating times, could be prepared and maintained under carefully controlled, laboratory conditions. However, the 0.05% nitric acid concentration is considered minimum for high speed, continuous, commercial operation.

The upper limit of nitric acid concentration is about 5% ms solution. This concentration permits a treating time o a out A second. Here also, although theoretically possible, for a commercial operation it would be, at least, impracticable to make and discontinue contact between the sheet and solution in a time interval of less than about /4 second. From the standpoint of controllability of both the nitric acid concentration of the solution and the treating time, a nitric acid content of about 0.5% by volume of aqueous solution, requiring a treating time of about from 2 to 3 seconds, is referred.

Although the rinsing step is critical and necessary, the nature of the rinsing medium is not critical. Any type of wash or rinse, hot or cold, such as plain water, distilled water or a slightly alkaline aqueous solution, that will provide the desired results, i.e. removal of the nitric acid treating solution and termination of its treating action before any etching of the plate occurs, may be used.

Any suitable method and/or apparatus for providing contact between the black iron sheet and aqueous nitric acid and for rinsing the treating solution from the plate after treatment may be used. For example, both the treating solution and the rinse may be sprayed onto the black iron sheet; or the sheet may be immersed first in a bath containing the aqueous nitric acid and then into a rinsing bath; or a combination of the above two methods may be used.

The following example is by way of explanation only and is not to be construed as a limitation on the inven' tion.

Example A portion (about /2) of a sheet of black iron was immersed in a room temperature solution of 1 m1. concentrated nitric acid in 200 ml. of water (0.5% by volume nitric acid) for 2 seconds and then withdrawn, immediately rinsed, with hot tap water and dried by passing hot air thereover. No etching was visible on the surface of the black iron. There appeared on the half of the sheet that was immersed, a pattern of brown to yellow streaks running roughly parallel to each other and at right angles to the line between the treated and untreated portions of the sheet.

After 14 days exposure to the atmosphere, the untreated portion of the sheet had rusted appreciably in a streaked pattern whereas the portion treated with the nitric acid solution showed only minor pin-points of rust. Further, at the dividing line between the treated and untreated portions, the streaks of rust ran directly into the stain streaks, while the relatively unrusted areas between the rust streaks ran directly into areas of the treated portion of the sheet which developed little or no stain. The heavier streaks of rust matched up with the stain streaks having the darker color.

These observations indicate not only that the aqueous nitric acid treatment of the instant invention inhibits atmospheric corrosion or rusting of black iron surfaces but also that it affords a means of predicting, before such rusting takes place, the areas of a black iron surface most susceptible to corrosion and where initial and maximum corrosion is likely to occur. The advantage of the former result is obvious. The advantage of the latter result-is in the nature of quality control whereby the corrodibility of a particular sheet or lot of black iron sheets can be determined after manufacture thereof. Knowing this fact, recommendations can be made by the steel manufacture as to the ultimate use and/or handling of the sheet.

Although the invention has been described with relation to low carbon steel or black iron in sheet form, it is within the purview of the instant invention to treat any shape or form of ferrous metal.

It is thought that the invention and many of its attendant advantages will be understood from the foregoing description, and it will be apparent that various changes may be made in the steps of the method described and their order of accomplishment without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the method hereinbefore described being merely a preferred embodiment thereof.

I claim:

1. A method of treating a ferrous metal surface comprising contacting said surface with an aqueous nitric :acid solution for a time varying inversely with the nitric acid concentration of said solution, said time and nitric acid concentration relationship being from about second at a maximum nitric acid concentration of 5.0% by volume to about seconds at a minimum nitric acid concentration of 0.05% by volume, and thereafter removing said solution from the said surface before etching of said surface occurs.

2. A method of treating a black iron surface comprising immersing said surface in a solution consisting of about 0.5% by volume concentrated nitric acid in water for about from 2 to 3 seconds, withdrawing said surface from said solution, and rinsing said solution from said surface before etching of said surface occurs.

3. The method set forth in claim 2 wherein said surface is dried after rinsing.

4. A method of treating the surface of black iron comprising immersing said surface in a solution consisting of about 0.5 concentrated nitric acid by volume in distilled water for about from 2 to 3 seconds, said solution and said surface being at room temperature during the immersion, withdrawing said surface from said solution, rinsing said solution from said surface with hot water before etching of said surface occurs, and drying said rinsed surface by passing hot air thereover.

5. The product resulting from the method set forth in claim 1 wherein said black iron surface exhibits a plurality of brown to yellow stains as a result of said treatment.

References Cited in the file of this patent UNITED STATES PATENTS 1,447,096 Martin Feb. 27, 1923 1,859,736 George May 24, 1932 1,902,815 Goodhue Mar. 18, 1933 2,410,322 Weesner et al. Oct. 29, 1946 2,773,623 Schuster June 1, 1954 2,777,785 Schuster Jan. 15, 1957 

1. A METHOD OF TREATING A FAERROUS METAL SURFACE COMIPRISING CONTACTING SAID SURFACE WITH AN AQUEOUS NITRIC ACID SOLUTION FOR A TIME VARYING INVERSELY WITH THE NITRIC ACID CONCENTRATION OF SAID SOLUTION, SAID TIME AND NITRIC ACID CONTRATION RELATIONSHIP BEING FROM ABOUT 1/4 SECOND AT A MAXIMUM NITRIC ACID CONCENTRATION OF 5.0% BY VOLUME TO ABOUT 60 SEOCNDS AT A MINIMUM NITRIC ACID CONCENTRATION OF 0.05% BY VOLUME, AND THEREAFTER REMOVING SAID SOLUTION FROM THE SAID SURFACE BEFORE ETCHING OF SAID SURFACE OCCURS. 